1. Field of the Invention
The invention pertains to the field of small optical display systems. More particularly, the invention pertains to apparatus and methods for enhancement of a real image projection system through the use of several combinations of methods of aberration reduction. The primary enhancement is reduced ghosting and reduction of astigmatism, common with small real image systems wherein the viewing distance is relatively close.
2. Description of Related Art
The present invention pertains to a real image projection system, and in particular, to a system in which an image of a real object is formed in space, giving the illusion that a real object exists at that point in space, when in reality it does not. Real image projection systems normally incorporate spherical or parabolic mirrors for imaging. In large systems, where the viewer is located at a significant distance from the image being viewed, optical aberrations, such as, for example, spherical aberrations, and astigmatism in particular, are not as much of a problem as in smaller systems, where the viewer is located close to the image. Astigmatism causes eye strain when viewing the image for long periods of time, and this has been one of the primary reasons that small real image projection systems have not been widely incorporated in gaming applications, as well as in workstation applications.
Another reason for the lack of wide-spread acceptance of small real imaging systems is that ghost images in the systems are much more noticeable, when viewing the display from a close distance. Many approaches have been used to reduce ghosting, including tinted beamsplitters and circular polarizers, none of which are extremely effective. Even with the use of a circular polarizing window, the ghost images are visible, although they can be significantly reduced. The circular polarizing windows typically have a maximum transmission of 42%, and this significantly reduces image brightness. Thus, in an arcade or other public area that is brightly lit, the real image usually is difficult to see.
Other optical aberrations present problems for real image projection systems. For example, field curvature distortion is a significant problem for smaller systems, because of the shorter focal lengths typically associated with small systems. For example, a rectangular shape displayed on a CRT screen projects as a xe2x80x9cfish-eyedxe2x80x9d real image of the target object. The sides of the rectangular image appear to bow outward and the center of the rectangular image appears magnified, as compared to the edges. This is a natural phenomenon of spherical mirrors, and cannot normally be corrected without a significant number of additional lenses in the beam-path, which makes the display system significantly larger in physical size, as well as making the cost of manufacturing such displays prohibitive.
Optics have been designed to compensate for some of these aberrations, such as, for example, spherical aberrations, through use of the Mangin mirror. This is a mirror that has a reflective convex spherical surface of longer radius, and a transmissive concave spherical surface of shorter radius. However, this approach is not practical for a real image projection system, because the image source or target is not a point at the focal point or center of curvature of the mirror, as in a single point imaging system. In a real image projection system, the target usually is a rectangle, such as a monitor screen, where only the center of the screen is on the axis or at the focal point of the mirror. The Mangin dual curve corrective mirror could be significantly improved by replacing the concave spherical surface with an aspheric surface of revolution, which will reduce the astigmatism for points offset from the axis of the mirror. Thus, a Mangin mirror incorporating two spherical curves is extremely effective for points along the axis of an on-axis system, but the problem of astigmatism becomes progressively worse as the target point deviates from the axis of the mirror curvature. An aspheric curve on the concave surface would optimize the correction and reduce the astigmatism for a larger area around the axis or focal point.
One other reason that small systems have not become mainstream is because of the difficulty in producing the curved optics in reasonable volume. The problem is compounded when corrective optical curvatures are incorporated.
The real image projection system of the present invention uses several combined methods of producing small displays having improved imagery and reduced ghosting over prior art systems.
The system of the present invention uses a single curved mirror having two different optical surfaces of revolution, one on the convex surface and one on the concave surface. In one embodiment, the convex surface is a conical curve of spherical or parabolic surface of revolution, coated with a reflective optical coating. The concave surface is much like that of a Mangin mirror, but it has an aspheric surface of revolution, optimized to reduce spherical aberrations over a larger area offset from the optical axis. The system optionally employs a single aspheric surface of revolution on the concave surface for reduction of aberrations, although the Mangin mirror approach, using an aspheric concave surface of revolution, is the preferred embodiment.
There are two problems in manufacturing mirrors with minimal aberrations and astigmatism. The quality of the surface of revolution must be very precise. This typically involves precision polishing of the surfaces, thus limiting the volume of such mirrors that can be produced in a given time, and creating a cost that is outside of what generally is acceptable for a commercial real image display system. Also, aspheres are extremely difficult to produce and must be hand polished to precise curves. Thus, the complexity of the aspheric optical surfaces of revolution prevents the mirror from being produced in large volume, therefore the preferred method of manufacture is injection molding.
Injection molding is a common manufacturing method for small optics, although it has not commonly been used for optics over 5 inches in diameter. Injection molding of optics up to 18 inches in diameter can be successfully accomplished through several techniques. Increasing the mold gate size by 200% over normal practice, and adding additional gates to the mold ensures uniform flow. It is also important to increase the number of venting ports for the displaced air during the injection process. Another step that is necessary is to calculate the press tonnage at 150% over the standard requirement.
The surface accuracy of the mirror is directly related to the surface accuracy of the mold. Creating a precision aspheric curve is not a simple task for an injection mold. The mold must be designed with separate inserts, or smaller steel blocks. A steel block is diamond-turned to the aspheric surface of revolution as a full parent optic or as a large bowl, whereby the optic is round and symmetrical. Diamond-turning is accomplished with a custom air-bearing lathe. The air bearings insure extremely precise accuracy, which is not possible with conventional lathes. The aspheric surface is cut with a single point diamond, and the curve accuracy is maintained using xe2x80x9cCNCxe2x80x9d or Computer Numeric Control. For visual optical systems, the accuracy of the curve should be held to a curvature rate change of 5 fringes per inch or 0.000055 inches (i.e., 55 millionths of an inch) surface deviation per linear inch across the entire surface of the mold. This is a much more liberal tolerance than laser optics or telescope optics would require, but still is well beyond the capabilities of convention machining. These tolerances can be held only through diamond-turning with air bearings. The machined surface of a diamond-turned curvature appears polished and requires minimum hand finishing.
Once the final curvature is cut into the steel block, the final shape of the optic is rough-cut slightly oversize with a diamond wire-saw, and, finally, the finished edge surfaces are EDM cut or xe2x80x9celectrical discharge machinedxe2x80x9d. The main mold housing is then machined to a tolerance in which the insert will fit into the housing with gaps less than that in which molten plastic will penetrate, or a nearly seamless fit.
The mirror surface must be free of stress and aberrations, thus, mounting design is extremely important. Acrylic mirrors stress and distort when held tightly in a fixture. Any mounting on the rear surface of the mirror will transmit distortion or an optical imprint to the front surface. Stress-free mounting requires that the mirror be held from the sides or edges, and the mounting tabs must be floating in a mounting slot with enough clearance so as not to flex the mirror in any way.
Field curvature distortion cannot be removed optically without incorporating several corrective lenses in the optical path. Field curvature distortion causes a target object to be projected with the appearance of a xe2x80x9cfish-eyexe2x80x9d lens. The center of the image appears magnified and the edges tend to bow outward. The present invention uses a unique approach to correcting for this distortion. The target image on the face of the CRT is xe2x80x9cPin-Cushionedxe2x80x9d or compensated for the fish-eye distortion. This is accomplished electronically in a CRT, or by software in the case of a LCD panel input.
The primary problem with small real imaging systems is the production of unwanted ghost images, which are typical of on-axis projection systems. When a viewer looks into the aperture of a real image projection system, the viewer sees a reflection of himself upside-down, floating inside the display unit. Any source of light or reflection outside the system, which enters the window view-aperture, forms an image inside the system, which is visible to the observer. The device of the present invention uses a combination of methods to eliminate ghosting. The curved mirror is tilted off-axis to the input beam-path, preferably at an angle between 10 and 20 degrees. The optimum tilt is 15 degrees, since this totally eliminates ghosting, while keeping field curvature distortion at a minimum that can be corrected using the xe2x80x9cpin-cushionedxe2x80x9d input source.
In the xe2x80x9ctilted off-axis mirrorxe2x80x9d configuration, the system of the present invention comprises a target object or real object, a beamsplitter positioned in the beam-path between the target object and a curved mirror, with the mirror being positioned off-axis to the normal beam-path axis. The beamsplitter is positioned substantially at, but not limited to, a 45-degree angle relative to the optical axis of the curved mirror. Light from the target object is directed in diverging rays, reflecting off a fold mirror, transmitting through the beamsplitter to the curved mirror. The primary beam-path axis, from the beamsplitter to the curved mirror, is at an angle non-coincident with the optical axis of the curved mirror. The light is reflected from the curved mirror in a convergent beam at a complementary angle to the primary beam-path angle, relative to the optical axis of the curved mirror. The converging beam-path then reflects off the beamsplitter and intersects, or comes to focus, at a point on the view axis, and forms a real image in space in front of the optical structure. Light entering the window aperture of the system is directed down and blocked from exiting within the viewing aperture, therefore, no ghost image is visible to the viewer.
The surface below the window aperture, inside of the system, preferably is a flat black surface. This surface is imaged at the window aperture, creating a very dark window opening, and providing extremely high contrast, when viewing the real image.
In a small real image display system, there is more chance for the viewer to move outside of the recommended view area or the xe2x80x9ceye-boxxe2x80x9d. As one looks at the system from below, at an angle looking upwards, there will be some ghosting visible. Such ghosting is reduced by using a neutral density window. A neutral density material is one that transmits or reflects an equal amount of light for all frequencies across the visible light spectrum. The optimum neutral density absorption is approximately 30%, although other absorption rates work as well, depending upon how the system is used and the required image brightness. Imaging light from the target is reduced by, for example, 30%, while light from a source outside the system passes through the neutral density window, reducing intensity by 30%, is reflected by the curved mirror, and is reduced by an additional 30% as it exits through the neutral density window, thus forming a ghost image of greatly reduced intensity. Another optional configuration is a neutral density beam-splitter, which perform the same function as the neutral density window.
Thus, in the present invention, the combination of (1) a precision, dual-curve, aspheric mirror (made possible through diamond-turning and injection molding), (2) a tilted off-axis configuration, and (3) a neutral density window or beamsplitter, provides a superior small real image projection system having a brighter image, significant ghost reduction, and significantly reduced optical aberrations and distortions, which are otherwise are common to small displays. When optional the pin-cushioned CRT input is incorporated, the system performance is improved even more. The incorporation of any or all of these improvements significantly improves the performance of small real image projection systems.